Basic steps to avoid slack chains and to optimize the run of the scraper chain on a face conveyor are known from German patent DE 2 017 949 A1 held by the applicant. In this citation it is stated that the sensor may theoretically be optical, electrical, magnetic or mechanical. In the exemplary embodiment of this citation a sensor is proposed that is fitted with a guide shoe which is steered by a lever and through which runs a section of the bottom race of the scraper chain behind the main drive of the scraper chain conveyor. The lever is pretensioned by means of a tensioning spring, and the deflection of the lever, which is dependent on the tension of the scraper chain in the bottom race, actuates a contact sensor, either one located above the lever or one located below it, such that depending on which contact transmitter has been triggered the tensioning drive for the return station will be extended or retracted.
From “Glückauf” 128 (1992), page 189, it is known that in particular on face conveyors the control of the tension of the scraper chain, especially the control of a dangling or slacking chain, is of major importance for operating a scraper chain conveyor safely and with avoidance of wear. If the chain is excessively pretensioned, an increased power demand will be observed, owing to increased friction throughout the complete system, and the vibratory stresses throughout the system may increase, which may result in chain fatigue and increased wear of all system components. If the chain is insufficiently pretensioned, the conveyor chain may disengage, slip, jam or jump out of the spoked chainwheel, which may cause damage to system components, including to the machine frame. In order to determine the tensions of the scraper chain, a measuring head is assigned both to the top race section of the scraper conveyor chain directly behind the auxiliary drive and to the bottom race section of the scraper conveyor chain directly behind the main drive. The signals of said measuring head are transmitted to an evaluation unit in order to extend or retract the tensionable return station, depending on the measurement signals. The measuring head assigned to the bottom race section detects the catenary and the measuring head located in the top race detects the chain geometry, in other words the distance of the vertical and horizontal chain links from one another.
In the prior art, the tension of the chain is generally determined indirectly, in particular with pressure sensors, travel sensors or counting sensors. As an alternative, direct determination of the chain tension by means of inductive proximity sensors, which are installed in the top race of the conveyor, is also known. The use of indirect measuring sensors in underground mining is critical, owing to the high inductances of the proximity sensors, and long-term reliability can not be achieved using the known proximity sensors.